In the field of bottling of liquids, a system is known comprising a feeding conveyor for feeding a succession of empty containers to a filling machine, in turn comprising a rotating conveyor (so called “carousel”), carrying a number of filling devices. Each filling device includes a filling valve, which is displaceable between an open positions, in which it allows the flow of a pourable product within the respective container, and a closed position, in which it prevents the pourable product from flowing within the container.
The filling devices are mounted to rotate continuously about a longitudinal axis, to engage the empty containers and fill the containers with the pourable product, coming from a product tank. In case of carbonated liquids, filling operations may include also feeding pressurized gas, such as carbon dioxide, into the containers to pressurize them, before filling the same containers with a carbonated liquid, and, afterwards, decompressing the filled containers.
In the processing plant, the containers may then be feed to a capping machine, which is coupled to the filling machine by at least one transfer wheel and which closes the containers with respective caps, and/or to other processing machines, such as labeling machines.
The tank containing the pourable product may be positioned on the carousel, or externally thereto, and it is fluidically connected to the filling devices by means of respective ducts. A flowmeter is arranged along each duct, to measure, when the respective filling valves are arranged in the open positions, the flow rate of fluid by which the containers are filled.
The measurement of the flow rate performed by the flowmeters is used by local control units associated with each filling device, to control actuation of the filling valves between the respective open and closed positions, so as to fill the containers with a desired volume of pourable food product and reach a desired and repeatable filling level within the same containers.
In particular, use of a vortex flowmeter (also known as vortex shedding flowmeter) has been proposed, to measure the flow rate of the pourable product reaching the filling devices; for example, document WO 2012/085828 A1, in the name of the present Applicant, discloses use of a vortex flowmeter in a filling system of a filling machine.
As schematically shown in FIG. 1, a vortex flowmeter 1 comprises a main tubular body 2, having a longitudinal axis A and designed to be arranged along the duct interposed between the tank and a respective filling device (here not shown); tubular body 2 defines an inlet mouth 2a and an outlet mouth 2b, designed to be coupled to the duct for passage of fluid.
An obstacle 4, e.g. having a trapezoidal axial section, is inserted in the tubular body 2, so as to define an impact surface 4a orthogonal to longitudinal axis A.
A sensor 5, in particular a piezoelectric sensor, is arranged within the tubular body 2, downstream the obstacle 4 proceeding from inlet mouth 2a towards outlet mouth 2b. 
When the pourable product passes through the passage defined by tubular body 2, it impinges upon impact surface 4a of obstacle 4, generating a train of vortexes 6 (usually known as Karman vortexes), the frequency of which is proportional to the speed of the pourable product.
Sensor 5 is configured to transform the oscillating pressure spikes Δp associated with vortexes 6 into an electrical quantity V, that may be processed to generate a detection signal, indicative of the flow rate of the pourable product.
In particular, the detection signal output by the vortex flowmeter 1 is a pulsed signal (a digital signal including a train of pulses, e.g. rectangular pulses), where each pulse (having rise and fall edges) corresponds to a detected pressure spike and an associated vortex 6; the frequency of the generated pulses corresponds to the frequency of vortexes 6, and so is substantially proportional to the flow rate.
Although advantageous in many respects (for example since it allows measuring flow rate also of fluids having a very low electrical conductivity, e.g. lower than 15 μS, contrary to other types of flowmeters, e.g. magnetic flowmeters), the present Applicant has realized that use of a vortex flowmeter to control operation of the filling devices also has some issues, at least in certain operating conditions.
For example, environment vibrations, e.g. machine vibrations, may influence the sensor 5, and cause generation of a number of spurious pulses (i.e. not related to vortex generation due to fluid passage).
Moreover, when the filling valves in the filling devices are actuated to close, or open, the fluid passage, burst pressure or back-flow may be generated in the duct, and therefore within tubular body 2 of vortex flowmeter 1. This burst pressure may generate a number of pulses, again not related to the flow of pourable product within the duct, which are nonetheless detected by sensor 5 of vortex flowmeter 1.
In general, the present Applicant has realized that computation of the amount of product flowing within the duct towards the filling devices, based on the output of a vortex flowmeter, may be subject to a number of errors (according to varying operating conditions), which may lead to corresponding errors in the determination of the level of product in the filled containers.
Moreover, due to the high speed and number of filling operations performed by the filling machine, real time compensation or correction of the above errors (during execution of the filling operations) may often prove to be a difficult task.